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Twitter @ahsan_danieL

Theme : Microsoft

Linux Operating System

Topics of 1st Assignment

• Page Transfer Management

• Thrashing and Remedy

• Reference String

• Reference Locality

• Working Set

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• Page Transfer

Management

• Thrashing and

Remedy• Reference String

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•Reference Locality • Working Set

Page Transfer Management

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In computer operating systems, paging is a memory

management scheme by which a computer stores and

retrieves data from secondary storage for use in main

memory. In this scheme, the operating system retrieves

data from secondary storage in same-

size blocks called pages.

Paging is an important part of virtual

memory implementations in modern operating systems,

using secondary storage to let programs exceed the size

of available physical memory.

Page Replacement Algorithms

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Page replacement algorithms are the techniques using

which Operating System decides which memory pages to

swap out, write to disk when a page of memory needs to

be allocated. Paging happens whenever a page fault

occurs and a free page cannot be used for allocation

purpose accounting to reason that pages are not available

or the number of free pages is lower than required pages.

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Thrashing and Remedy

After completing initialization, most programs operate on a small number of code and data pages

compared to the total memory the program requires.

The pages most frequently accessed are called the working set.

Thrashing occurs on a program that works with huge data structures, as its large working set causes

continual page faults that drastically slow down the system

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Reference String

The latter choice produces a large number

of data, where we note two things :

For a given page size we need to consider only the page number, not the

entire address.

If we have a reference to a page p, then any immediately following

references to page p will never cause a page fault. Page p will be in

memory after the first reference; the immediately following references

will not fault.

For example :

consider the following sequence of addresses – 123 , 215 , 600 , 1234

If page size is 100 then the reference string is 1 , 2 , 6 , 12.

Reference String

Definition

The string of memory references is

called reference string. Reference

strings are generated artificially or by

tracing a given system and recording

the address of each memory

reference.

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Reference Locality

Reference Locality

Spatial locality Branch locality

Increase own agility Specific security controls

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In computer science, locality of reference, also known as the principle of locality, is a term for the

phenomenon in which the same values, or related storage locations , are frequently accessed.

Types of locality

Temporal locality

Increase own agility

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Reference Locality Discussing about the types of locality.

SLIf a particular memory location is referenced at a particular time, then it is likely that nearby memory

locations will be referenced in the near future. In this case it is common to attempt to guess the size and

shape of the area around the current reference for which it is worthwhile to prepare faster access.

BLIf there are only a few possible alternatives for the prospective part of the path in the spatial-temporal

coordinate space. This is the case when an instruction loop has a simple structure, or the possible outcome of

a small system of conditional branching instructions is restricted to a small set of possibilities.

TL

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Working Set

Working Set

After completing initialization, most programs operate on a

small number of code and data pages compared to the total

memory the program requires. The pages most frequently

accessed are called the working set.

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Cache Coherence

Cache Concurrency

Cache Coherence

In computer science, cache coherence is the consistency of shared resource data that

ends up stored in multiple local caches. When clients in a system maintain caches of a

common memory resource, problems may arise with inconsistent data, which is

particularly the case with CPUs in a multiprocessing system.

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Cache Coherence [Detail]

shared memory multiprocessor

separate cache memory for each processor

possible to have many copies of any one instruction operand

one copy in the main memory and one in each cache memory.

When one copy of an operand is changed, the other copies of the operand must be

changed also.

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Cache Coherency

Consistency means that each user sees a consistent view of the data, including the

visible changes made by the users? own transactions, as well as the transactions of

other users. Oracle automatically supplies a query with read-consistent data, so that all

data that the query sees comes from a single point in time (statement-level read

consistency).

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In computing, cache coherence (also cache coherency) refers to the consistency of

data stored in local caches of a shared resource. Cache coherence is a special case of

memory coherence. When clients in a system maintain caches of a common memory

resource, problems may arise with inconsistent data.

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Thank you